1. Field of the Invention
The present invention relates to a sheet detecting apparatus, a sheet conveying apparatus and an image forming apparatus to improve detecting accuracy of a sheet.
2. Related Background Art
[Background Art 1]
In the past, as a sheet sensor (sheet position detecting means) 1 used for conveyance control of a sheet S in an image forming apparatus and the like, for example, as shown in FIG. 27, a sensor constituted by a photo-interrupter 3 and a mechanical flag 2 contacted with the sheet S conveyed in a sheet conveying direction indicated by the arrow is well known.
[Background Art 2]
Further, in such a sheet sensor of contact type, although disadvantages such as chattering and time lag upon contacting of the sensor with the sheet may occur, when the conveying speed of the sheet is relatively low, since such disadvantages do not arise any problem, such a sheet sensor has been used in many apparatuses.
However, when the above-mentioned disadvantages arise serious problem or when the sheet conveying speed is high, detection of the sheet S is effected by using a sensor 4 of reflection type comprised of a light emitting element 5 and a light receiving element 6 as shown in FIG. 28.
[Background Art 3]
In an image forming apparatus such as a copying machine handling a sheet, in many cases, exclusive sheets (transfer materials) recommended by the maker of the image forming apparatus are used. However, in the present days in which various user needs exists, it has been requested that various kinds of sheets be used as transfer materials for the copying machines, other than the sheets recommended by the maker.
In the past, regarding such various sheets, the sheet has been conveyed under sheet conveyance control (for example, conveying timing upon registration and/or conveying speed) similar to the exclusive sheet. Alternatively, for example, when a thick sheet is used, in some cases, user or service man has effected the setting of the thick sheet with respect to a predetermined sheet tray through an operating portion of the copying machine to change the conveying timing.
However, in the above-mentioned background art 1, if any play is generated between the mechanical flag 2 and a shaft as a fulcrum 7 for the flag for a long term use or if an operating point is changed in dependence upon a conveying condition (for example, position along a thickness direction in a conveying path) of the sheet to be detected, error of sheet detecting accuracy of aboutxc2x12 to 3 mm may occur.
In the sensor 4 of reflection type in the background art 2, depending upon the kind of the sheet, detecting error of about 1.3 mm occurred in the range of a detecting point (fully described in connection with first embodiment shown in FIG. 5).
Further, in the sensor of reflection type, detecting error due to a sheet passing position (distance between the sensor of reflection type and the sheet) may cause serious problems.
FIGS. 29A and 29B are explanatory views showing a relationship between a height direction distance with respect to the sheet (this height direction distance can be considered as deviation in the sheet thickness direction when the sheet is passed through the conveying path or as change in distance with respect to the sensor caused by the thickness of the sheet itself) and output of a sheet position detecting sensor, where FIG. 29A shows output OP of the sheet position detecting sensor with respect to the height direction distance h, and FIG. 29B shows outputs OP of the sheet position detecting sensor with respect to a passing time t when the sheet is passed through the sheet position detecting sensor with heights h1 and h2.
As shown in FIGS. 29A and 29B, for example, when it is assumed that a timing that the sheet reaches the sheet position detecting sensor corresponds to a timing that the output OP of the sheet position detecting sensor becomes 01, the arrival detection timing becomes t1 when the sheet height is h1 and becomes t2 when the sheet height is h2.
In many cases, although such detecting errors normally do not arise any serious problem, when the sheet conveying speed is requested to be further increased or when the conveying accuracy is requested to be further improved, such errors cannot be neglected.
In the above-mentioned background art 3, regarding various kinds of sheets, when the sheet is conveyed at the uniform conveying timing, since a load and a friction force acting on conveying means are differentiated depending upon the kind of sheet, poor sheet conveyance may occur.
Particularly, in a process for forming a loop in a leading end portion of the sheet at a registration portion to effect correction of skew-feed of the sheet and registration of the leading end of the sheet with an image, due to dispersion in sheet stopped positions depending upon the difference in sheet kinds, loop amounts in the sheets are differentiated, with the result that function of the registration means may be worsened and sheet jam may occur.
Further, when the user or service man effects the setting of the thick sheet with respect to the predetermined sheet tray through the operating portion of the copying machine to change the conveying timing, not only operability and workability may be worsened but also it may be difficult to cope with various sheets.
The present invention aims to eliminate the above-mentioned conventional drawbacks and an object of the present invention is to make higher accurate sheet detection possible thereby to improve reliability of stable sheet conveyance under high speed sheet conveyance and small sheet-to-sheet distance control by reducing a detecting error due to difference in sheet kind and by reducing a detecting error due to difference in sheet conveying position (distance with respect to a sensor of reflection type), when the sensor of reflection type is used as means for detecting the fact that a sheet being conveyed reaches a predetermined position.
Another object of the present invention is to improve reliability of stable sheet conveyance by making proper sheet conveyance control possible for various sheets without worsening operability and workability of users and by detecting the fact that a sheet is deviated from an allowable value permitting stable sheet conveyance and by effecting sheet conveyance control on the basis of a detecting result.
To achieve the above object, a sheet detecting apparatus according to the present invention comprises sheet position detecting means for detecting a fact that a sheet being conveyed in a conveying path reaches a predetermined position, sheet surface detecting means for detecting a surface condition of the sheet, and correction means for correcting a detection result of the sheet position detecting means in accordance with the surface condition of the sheet detected by the sheet surface detecting means.
Further, the present invention may provide a sheet detecting apparatus comprising sheet position detecting means for detecting a fact that a sheet being conveyed in a conveying path reaches a predetermined position, sheet height detecting means disposed in the vicinity of the sheet position detecting means and adapted to detect a distance between the sheet position detecting means and the sheet in a direction perpendicular to a sheet conveying plane, and correction means for correcting a detection result of the sheet position detecting means in accordance with the distance detected by the sheet height detecting means.
Further, the present invention may provide a sheet detecting apparatus comprising sheet position detecting means for detecting a fact that a sheet being conveyed in a conveying path reaches a predetermined position, sheet surface detecting means for detecting a surface condition of the sheet, sheet thickness detecting means for detecting a thickness of the sheet, and correction means for correcting a detection result of the sheet position detecting means in accordance with a detection result of the surface condition of the sheet detected by the sheet surface detecting means and the thickness of the sheet.
The sheet position detecting means includes a light emitting element for illuminating light onto the sheet being conveyed, and a light receiving element for receiving light reflected from the sheet, and, when an output value of the light receiving element for converting the received light into an electric signal exceeds a predetermined threshold level, a sheet detection signal is emitted, and the correction means preferably correct a sheet detecting timing of the sheet position detecting means.
It is preferable that timer means counted in association with the output of the sheet position detecting means is provided, and the correction means correct a set value of the timer means.
Further, it is preferable that the sheet position detecting means emit a sheet detection signal when the output value of the light receiving element for converting the received light into the electric signal exceeds the predetermined threshold level, and the correction means alters the threshold level.
Further, it is preferable that the sheet surface detecting means includes a light emitting element for illuminating light onto the sheet, polarization separation means for separating light reflected from the sheet into two polarization components, and light receiving elements for receiving the separated two polarization components respectively, and the surface condition of the sheet is detected on the basis of a calculation value obtained from output values of the two polarization components.
The calculation value may be a value obtained by dividing a difference between the two polarization components by sum of the two polarization components.
Judging means for judging as sheet judgement error if the calculation value is deviated from a predetermined range may be provided.
The sheet surface detecting means may detect reflectance with respect to the sheet.
It is preferable that the sheet surface detecting means are disposed at an upstream side of the sheet position detecting means in the conveying path.
In a sheet conveying apparatus according to the present invention, the above-mentioned sheet detecting apparatus is provided in the conveying path through which the sheet is conveyed by conveying means, and conveyance control means for controlling the conveying means on the basis of the corrected detection result of the sheet position detecting means is provided.
The conveyance control means may alter conveyance distance between the sheets given by the conveying means.
The conveyance control means may alter a sheet conveying speed of the conveying means.
The conveyance control means may effect control to alter a sheet stopping timing when the sheet is stopped by the conveying means.
It is preferable that the sheet stopping timing is a sheet stopping operation of the conveying means when a loop is formed in the sheet by abutting the leading end of the sheet against a registration member (disposed in the conveying path) by the conveying means or a conveyance starting operation for conveying the sheet from the registration member.
Further, in a sheet conveying apparatus according to the present invention, the above-mentioned sheet detecting apparatus is provided in the conveying path through which the sheet is conveyed by conveying means, and a sheet stopping timing associated with a sheet stopping operation of the conveying means or a conveyance starting operation (for conveying the sheet from the registration member) when a loop is formed in the sheet by abutting the leading end of the sheet against a registration member (disposed in the conveying path) by the conveying means is altered on the basis of the corrected detection result of the sheet position detecting means and kind and thickness of the sheet detected by the sheet surface detecting means and the sheet thickness detecting means and loop forming amount control information corresponding to pre-set kind and thickness of sheet.
The sheet stopping timing may be altered so that the loop forming amount becomes constant.
It is preferable that the sheet stopping timing associated with the sheet stopping operation of the conveying means or a conveyance starting operation (for conveying the sheet from the registration member) is altered so that the loop forming amount control information corresponding to pre-set kind and thickness of sheet becomes loop forming amount corresponding to sheets having different kind or thickness.
An image forming apparatus according to the present invention comprises the above-mentioned sheet conveying apparatus, and image forming means for forming an image on the sheet conveyed by the sheet conveying apparatus.
With this arrangement, sheet detecting accuracy can be enhanced, and the reliability of stable sheet conveyance under high speed sheet conveyance and small sheet-to-sheet distance control can be achieved.
Further, the proper sheet conveyance control for various sheets can be performed, thereby achieving the stable sheet conveyance.